Things You'll Need
Large pot of water
Oven with range top
To shrink a hat to better fit to your head, wear it in a hot shower and then let it dry in the hot sun. Even if your hat-shrink is a total success, the hat likely won't look as good as one that's purchased in the correct size.
Fitted caps can become a bit snug after a run through the wash. That's because hot water combined with the heat of a dryer can shrink polyester. Heat is the secret to shrinking a polyester hat. It's a good thing to keep in mind when washing that old, dirty ball cap but it can also come in handy to know how to shrink a cap if you're considering getting a significant hair cut or have lost some of those bushy locks you sported during your glory days on the baseball diamond.
Soak the hat in the sink. Use warm water like you would to do the dishes. Wash the hat by hand with a pinch of laundry detergent. Any brand you normally use for the rest of your wardrobe will work. Remove the hat from the water and shake off the excess water after about five minutes soaking.
Preheat the oven to 300 degrees.
Boil water in a pot large enough to fully submerge the hat.
Dunk the hat in a rolling boil for about 10 minutes. Leave the lid off the pot. Remove the hat with a pair of tongs. Shake the hat over the sink until it is no longer soaked but still damp.
Place the hat in the oven for five minutes. Cool for a moment on a rack, just like you would a batch of cookies.
Run the hat through the dishwasher if it needs to shrink more. Place it on the top rack and do not fill the soap reservoir. Use the standard dish setting and run the hat through the full cycle to let the hot drying process work on the polyester.
Dry the hat in the sun.